Vibrating percussion bit

ABSTRACT

A percussion bit for hard rock formations includes a body having a keyhole shaped slot formed through it which causes the body to vibrate laterally as a tuning fork whenever the bit is impacted. Because of the vibration of the body, hardened inserts mounted in the body form a borehole not only by vertical percussive engagement but also by lateral percussive engagement due to the tuning fork action of the bit and by scraping due to rotation of the bit in the borehole. Passageways extending through the body direct compressed air through the bit to remove drilling debris from the borehole.

0 United States Patent 11 1 1111 3,738,435 Bronson June 12, 1973 [54] VIBRATING PERCUSSION BIT 2,102,236 12/1937 .lohansen 175/56 2,970,660 2 196i B d' ,J I75 56 [75] Inventor! Charles DallasTex' 3,388,756 6/1968 Vir i ail. 175/110 [73] Assignee: Vorel Manufacturing Company, Dallas, Tex. Primary Examiner-James A. Leppink Att0rneyRichards, Harris & Hubbard [22] Filed: July 29, 1971 Related Application Data A percussion bit for hard rock formations includes a Continuation-impart 0f y 7, body having a keyhole shaped slot formed through it 1969, abandonedwhich causes the body to vibrate laterally as a tuning fork whenever the bit is impacted. Because of the vi- [52] US. Cl 175/56, 175/410, 175/415 bration of the body, hardened inserts mounted in the [51] Int. Cl E21!) 5/00, E216 13/01 body form a borehole not l by vertical percussive [58] Field of Search 175/55, 56, 410, engagement but also by lateral percussive engagement 175/414, 415420 due to the tuning fork action of the bit and by scraping due to rotation of the bit in the borehole. Passageways [56] References C'ted extending through the body direct compressed air UNlTED STATES PATENTS through the bit to remove drilling debris from the bore- 1,561,580 11/1925 Watson 175/414 hole- 5s3,35s 5/1897 Carrigg..... 175/414 730,786 6/1903 Murphy 175/414 16 Clams, 7 Flgures 1,540,047 6/1925 Watson..... 175/414 1,540,048 6/1925 Watson 175/414 Pms'mmwm 3.738.435

SHEET 1 0F 2 FIG. I FIG; 2

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INVENTOR 26 CHARLES 1.. BRONSON FIG. 3

ATTORNEY PAIENIED J11"! 3 3. 738 .435

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FIG 5 F ICE. 6

INVENTOR CHARLES L.BRONSON A TTORNE Y5 CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 847,484, filed July 7, 1969, now abandoned.

BACKGROUND OF THE INVENTION In the drilling industry, both rotary and percussion bits are employed. Typically, percussion bits are operated by repeatedly impacting the formation being drilled with relatively low frequency (i.e., 1,500 cycles per minute), long strokes, high energy blows. This action crushes and spalls the formation to form a borehole. This invention relates to a percussion bit in which portions of the bit are vibrated relative to each other as the bit is impacted. The vibration adds relatively high frequency (i.e., 15,000 cycles per second) short stroke, low energy blows to the conventional drilling blows. Preferably, the vibration occurs at right angles to the conventional stroke.

SUMMARY OF THE INVENTION In the preferred embodiment, this invention comprises a bit including a body having a slot formed in it for vibrating a portion of the body relative to another portion whenever the bit is impacted. Preferably, the slot extends along the direction of impact so that the portions of the body vibrate relative to each other in a direction perpendicular to the direction of impact.

More particularly, in accordance with the preferred embodiment of the invention, a percussion bit for earth boring or the like includes a metal body having a longitudinal axis and comprising a shank and a right circular cylindrical head extending from the shank to a face disposed generally normal to the axis. A plurality of hardened inserts are secured in the head and project outwardly from the face and are so arranged that upon rotation of the head the inserts sweep the entire area of the face. A slot extends axially into the head from the face and has a depth equal to between one-quarter and one-half of the diameter of the head. The slot is sufficiently narrow that cuttings formed during the operation of the bit cannot enter the slot. Thus, in response to impacting of the bit in the axial direction, the portions of the head on opposite sides of the slot vibrate in the manner of a tuning fork to increase the cutting rate, increase bit life, and tend to form a borehole which is slightly larger in diameter than the largest di ameter of the head.

DESCRIPTION OF THE DRAWING A more complete understanding of the invention may be had by referring to the following Detailed Description when taken in conjunction with the Drawing, wherein:

FIG. 1 is a top view of a percussion bit comprising a first embodiment of the invention;

FIG. 2 is a sectional view taken along the line 2-2 in FIG. 1 in the direction of the arrows;

FIG. 3 is a sectional view taken along the line 33 in FIG. I in the direction of the arrows;

FIG. 4 is a sectional view taken along the line 4-4 in FIG. 1 in the direction of the arrows;

FIG. 5 is a top view of a percussion bit comprising a second embodiment of the invention;

FIG. 6 is a sectional view taken generally along the line 66 in FIG. 5 in the direction of the arrows; and

FIG. 7 is an illustration of the use of the invention.

DETAILED DESCRIPTION Referring now to the drawing, and particularly to FIG. 3 thereof, there is shown a percussion bit 10 comprising a first embodiment of the invention. The bit includes a body 12 formed from a high quality steel and comprised of a tubular shank l4 and a head 16. The tubular shank 14 is provided with suitable means (not shown), such as drive splines, internal or external threads, etc. for securing the bit 10 to the mandrel or stem of a conventional cable tool rig or pneumatic percussion drill.

The head 16 includes a central concave face portion 18 which extends outwardly from the center of the bit 10 and a peripheral conically tapered portion 20 which extends outwardly from the face portion 18. The head 16 also includes a slightly tapered cylindrical portion 22 which extends from the conically tapered portion 20 to the main portion 24 of the body 12 of the bit 10.

As is best shown in FIGS. 2 and 3, a cylindrical fluid inlet 26 extends through the body 12 along the axis of the bit 10. A first pair of fluid courses 28 extend from the inlet 26 to the face 18 of the head 16 of the bit 10. A second pair of fluid courses 30 extend from the inlet 26 to a pair of fluid return passageways 32 formed in the head 16 and the main portion 24 of the body 12. As is best shown in FIG. 2, the return passageways 32 are each comprised of a generally V-shaped side wall 34 and a tapered bottom wall 36.

Referring now to FIGS. 1 and 4, a plurality of hardened inserts 38 are mounted in the head 16 of the bit 10.-The inserts 38 are preferably formed from any of the various hard carbide materials or from any other material that is substantially harder than the steel used to form the body 12 and may be retained in the bit 16 by conventional press fitting techniques.

As is best shown in FIG. 1, some of the inserts 38 are mounted in the face portion 18 of the head 16 while the remainder of the inserts 38 are mounted in the tapered portion 20 of the head 16. As is shown in FIG. 4, each insert 38 includes a protruding face 40 and a cylindrical main body 42. The inserts 38 have been omitted from FIGS. 2 and 3 for clarity, however, it should be understood that the mounting of each of the inserts 38 shown in FIG. 1 is similar to the mounting of the inserts 38 illustrated in FIG. 4.

Referring again to FIGS. 1, 2 and 3, a cylindrical hole 44 extends transversely through the main portion 24 of the body 12 of the bit 10. A narrow slot 46 extends axially from the hole 44 to the face 18 of the head 16 of the bit 10. As is best shown in FIG. 1, the slot 46 extends across the entire width of the head 16.

The hole 44 and the slot 46 form a keyhole shaped slot through the bit 10 which causes the bit 10 to act as a tuning fork. That is, whenever the bit 10 is driven into engagement with the bottom of a borehole, the portions of the head 16 positioned on opposite sides of the slot 46 vibrate very rapidly with respect to each other. This action moves the inserts 38 of the bit 10 very rapidly along paths extending perpendicularly both with respect to the axis of the bit 10 and with respect to the slot 46. Thus, the hardened inserts 38 operate not only by vertical percussive engagement in the manner of a conventional percussion bit, but also by lateral percussive engagement due to the tuning fork action of the bit.

The bit is operated to form a borehole by holding the bit 10 against the bottom of the borehole and impacting the bit at a high rate. The bit 10 may be simultaneously rotated either by rifling or by a separate rotary drive mechanism. The repeated impacting of the bit 10 causes the inserts 38 to crush and spall the rocks or other matter through which the borehole is formed. During this action, the hole 44 and the slot 46 cause the portions of the head 16 of the bit 10 positioned on opposite sides of the slot 46 to vibrate rapidly toward and away from each other. The vibration of the portions of the head 16 causes the inserts 38 to spall hard rock formations through which the borehole is being formed.

Thus, the bit 10 functions like a conventional percussion bit with percussion energy also being directed horizontally to assist in spalling the rock formation.

During the operation of the bit 10 the inserts 38 positioned in the conically tapered portion 20 of the head 16 form the side walls of the borehole while the inserts 38 positioned in the face portion 18 of the head 16 act upon the bottom of the borehole. This action causes the inserts 38 to wear away. However, the relatively soft steel used to form the body 12 of the bit 10 wears away at least as fast as the hard material used to form the inserts 38. Therefore, the inserts 38 are continually exposed in the manner illustrated in FIG. 4 throughout the use of the bit 10. The shape of the inserts 38 illustrated in FIG. 4 does not change materially during the use of the bit 10 and, accordingly, the bit 10 need not be removed from the borehole for sharpening. The added resilience of the split body tends to limit the maximum crushing pressure and therefore reduces fracturing of the hardened inserts.

As the borehole is formed by the bit 10, a fluid such as compressed air is directed through the fluid inlet 26. The fluid passes through the fluid courses 28 and 30 up to the bottom of the borehole. As it enters the borehole, the fluid entrains particles of material loosened by the inserts 38 of the bit 10 and sweeps them out of the borehole through the return passageways 32. This action keeps the borehole free of drilling debris so that the bit 10 acts to increase the depth of the borehole rather than to merely crush and grind the particles already loosened. I

The percussion bit illustrated in FIGS. l-4 is superior to prior bits because the hole and the slot extending through the bit cause the bit to act as a tuning fork. This causes the inserts of the bit to form a borehole not by percussive action in the manner of conventional percussion bits, but also by lateral percussive action due to tuning fork vibrations.

Referring now to FIGS. 5 and 6, there is shown a percussion bit 50 comprising a second embodiment of the invention. The bit 50 includes a metal body 52 formed from high quality steel and comprised of a tubular shank 54 and a head 56. The tubular shank 54 is provided with suitable means for securing the bit 50 to the mandrel of a cable tool rig, a pneumatic percussion drill, or the like.

The head 56 comprises a right circular cylinder and extends to a face 58 which extends generally transversely across the distal end of the head 56. More particularly, the face comprises an outwardly facing conically tapered annular portion 60, a transversely extending portion 62, an inwardly facing conically tapered annular portion 64, and a recessed portion 66.

A plurality of hardened inserts 68 formed from tungsten carbide or the like are secured in the head 56 and extend outwardly from the face 58. As is best shown in FIG. 5, the inserts 68 may be considered as comprising four groups, including inserts 68a mounted in the outwardly facing annular portion 60 of the face 58, inserts 68b mounted in the transversely extending portion 62, inserts 68c mounted in the inwardly facing annular portion 64 and inserts 68d mounted in the recessed portion 66. The inserts 68 of the bit 50 are arranged so that upon rotation of the bit 50, the inserts 68 will sweep the entire area of the face 58.

A plurality of fluid passageways 70 extend through the head 56 of the bit 50 from the interior of the tubular shank 54 to outlets 72 formed in the face 58. The face 58 has a plurality of recesses 74 formed in it which extend from the outlet 72 to a plurality of return pas sageways 76. In the use of the bit 50 compressed air is directed through the tubular shank S4 and the fluid passageways 70 and is discharged from the outlets 72. The fluid then passes through the recesses 74 in the return passageways 76 to sweep cuttings out of the bottom of a borehole and upwardly to the annulus around the tubular shank 54 and hence out of the borehole. By this means, the bit 10 functions to continually form new cuttings, rather than to merely crush and pulverize cuttings previously formed.

The head 56 of the percussion bit 10 is divided into opposed portions of substantially equal mass by a slot .78 formed along the line extending parallel to but displaced from the longitudinal axis of the bit 50. As is best shown in FIG. 6, the innermost end of the slot comprises a stress relieving hole which extends transversely through the head 56. In the preferred embodiment of the invention, the slot 78 comprises a saw slot and the hole 80 comprises a drilled hole. However, those skilled in the art will realize that since the hole 80 is merely a stress relieving means, it may be of any desired shape and may be formed by any convenient technique.

In use, the percussion bit 50 is repeatedly impacted into engagement with formations of the earth and is preferably simultaneously rotated about its longitudinal axis. By this means the hardened inserts 68 are percussively engaged with the formations and thereby form a borehole of progressively increasing depth. It will be understood that because of the arrangement of the hardened inserts 68 in the face 58, rotation of the bit 50 causes the inserts to engage the entire bottom surface of the borehole. Thus, the depth of the borehole is increased uniformly over the entire area of the borehole.

The repeated impacting of the bit 50 with the bottom of the borehole also causes the portions of the head 56 on opposite sides of the slot 78 to vibrate in a direction extending mutually perpendicularly to the axis of the bit 50 and to the slot 78. This vibration is in the nature of the vibrations of the tines of a tuning fork and serves to percussively engage the hardened inserts 68a with the side walls of the borehole. By this means the borehole tends to be formed to a diameter which is larger than the largest diameter of the head 56.

It has been found that the operation of the bit 50 is enhanced if the slot 78 has a depth equal to between one-quarter and one-half of the diameter of the head 56. The greater the depth of the slot 78, the greater the amplitude of the vibratory movement of the bit. However, if the depth of the slot 78 is substantially greater than one-half of the diameter of the head 56, the portions of the head 56 on opposite sides of the slot 78 tend to vibrate into engagement with each other as the bit 50 is impacted. This tends to reduce the vibratory action of the bit so that the beneficial results that are otherwise obtained are reduced. Also, if the slot 78 is too deep, the portions of the head 56 on opposite sides of the slot 78 tend to wedge together as the periphery of the bit impacts the rounded corners of the bottom of the borehole. This sometimes results in both a loss of vibratory action and a loss of hole gage. On the other hand, if the depth of the slot 78 is substantially less than one-quarter of the diameter of the head 56, the amplitude of the lateral vibrations of the portions of the bit 50 which lie on opposite sides of the slot 78 is reduced to a point that the beneficial results of the vibratory characteristics of the bit are substantially eliminated. Thus, the use of a slot having a depth equal to between about one-quarter and about one-half of the diameter of the head is believed to be of significant importance to the practice of the invention.

The width of the slot 78 is also very important to the successful operation of the percussion bit 50. Thus, the slot 78 must be sufficiently wide that the portions of the head 56 on opposite sides of the slot do not engage one another during the vibratory action of the bit. On the other hand, the slot must be sufficiently narrow that cuttings formed during the operation of the bit do not enter the slot. In actual practice, it has been found that the width of the slot should be between one thirtysecond and one-sixteenth inch. This dimension does not vary materially with the diameter of the head, however, since percussion bits normally have a diameter of several inches or larger, it will be understood that the slot 78 is about two orders of magnitude narrower in width than the diameter of the head.

The operation of a percussion bit constructed in accordance with the present invention will be better understood by referring to FIG. 7. A bit 82 which is preferably constructed in accordance with the embodiment of the invention illustrated in FIGS. 1-4, 5 and 6 is secured to the lower end of the mandrel 84. A hammer 86 is employed to repeatedly impact the mandrel 84 and thereby repeatedly impact the bit 82 into engagement with formations of the earth. At the same time, a belt 86 or other means is employed to slowly rotate the mandrel and thereby rotate'the bit about its longitudinal axis. The repeated percussive engagement of the bit with the earth forms a borehole 90. The portions of the head of the bit on opposite sides of the slot vibrate toward and away from each other in a direction normal to the axis of the bit and to the slot in response to the impacting of the bit with the earth. A force applied to one portion of the head causes the other portion of the head to vibrate with generally equal amplitude in the manner of a tuning fork. This tends to increase the rate of penetration of the bit and increase the life of the bit. Although preferred embodiments of the invention have been described in the foregoing specification and illustrated in the accompanying Drawings, it will be understood that the invention is not limited to the embodiments disclosed but is capable of rearrangement, modification, and substitution of parts and elements without departing from the spirit of the invention.

What is claimed is:

1. In a percussion earth boring system, a bit comprising a metal body having a longitudinal axis and including a shank and ahead, said head comprising a substantially circular cylinder and including a portion secured to the shank and a bifurcated portion extending therefrom to a face disposed generally normal to the axis, said bifurcated portion having an axial length equal to between one-quarter and one-half of the diameter of.

the head.

2. The percussion earth boring system according to claim 1 further including means for repeatedly impacting the bit into engagement with formations of the earth and thereby generating cuttings and wherein the distance separating the bifurcations of the head is sufficiently narrow that the cuttings cannot enter the slot extending therebetween.

3. The percussion earth boring system according to claim 2 further including means for rotating the bit during the operation of the impacting means and a plurality of hardened inserts extending outwardly from the face and so arranged that upon rotation of the bit the inserts contact the entire bottom surface of the borehole.

4. The percussion earth boring system according to claim 3 further including fluid course means for directing a fluid through outlets formed in the face and thereby remove cuttings from the bottom of the borehole.

5, In a percussion bit for earth boring and the like, the combination of:

a right circular cylindrical head having a longitudinal axis and terminating in a face disposed generally normal to the longitudinal axis, and

a slot extending through the distal end of the head and parallel to the longitudinal axis, said slot being narrower in width than the diameter of the head by about two orders of magnitude and having a depth equal to between one-quarter and one-half of the diameter of the head.

6. The combination according to claim 5 further including a plurality of hardened inserts extending outwardly-from the face and so arranged that upon rotation and impacting of the head in a borehole the inserts are engaged with the entire bottom surface of the borehole.

7. The combination according to claim 6 further comprising fluid course means formed in the head for directing a fluid into the bottom of the borehole and thereby removing cuttings from the borehole.

8. The combination according to claim 7 wherein the slot divides the distal end of the head into proportions of substantially equal mass and wherein the innermost portion of the slot comprises stress relieving means.

9. In a percussion bit, the combination of:

a relatively soft metal body having a head portion, a tubular shank portion and a longitudinal axis, the head portion having a face disposed generally normal to the longitudinal axis;

a plurality of hardened inserts secured in the head and projecting outwardly from the face, said inserts being arranged such that upon rotation of the head the inserts sweep the entire area of the face; and

a slot formed in the head and extending parallel to but displaced from the longitudinal axis, said slot being sufficiently narrow to prevent the entry of cuttings of substantial size into the slot and having a total depth of between one-quarter and one-half the diameter of the head.

10. The combination according to claim 9 wherein the slot formed in the head comprises a saw cut extending transversely through the head and wherein the innermost portion of the slot comprises stress relieving means.

11. The combination defined in claim 10 further characterized by fluid course means extending through the head for directing fluid from within the tubular shank portion through outlets formed in the face and thereby removing cuttings from the bottom of a borehole.

12. The combination according to claim 11 wherein the face includes an outwardly facing conically tapered peripheral portion and wherein at least a portion of the hardened inserts are positioned at spaced points around the peripheral portion for outward movement with the portions of the head on opposite sides of the slot to form the side walls of the borehole.

13. A percussion drilling system comprising:

a metal body having a longitudinal axis and including a right circular cylindrical head and a generally transversely extending face comprising the distal end of the head;

a plurality of hardened inserts secured in the head and extending outwardly from the face, said inserts being so arranged that upon rotation of the head the inserts sweep the entire area of the face including the center;

a slot extending axially into the head from the face to a depth of between one-quarter and one-half the diameter of the head and extending entirely through the head transversely thereof; and means for repeatedly impacting the head with the earth and simultaneously rotating the head and thereby forming a borehole by percussive engagement of the hardened inserts with formations of the earth, said slot being sufficiently narrow that intact portions of the formation tend not to enter the slot,

whereby the impacting of the head causes the portions of the head on opposite sides of the slot to vibrate in a direction extending perpendicularly to the slot and to the axis so that the borehole is greater in diameter than the maximum diameter of the head.

14. The percussion drilling system according to claim 13 wherein the slot is about two orders of magnitude narrower in width than the diameter of the head.

15. The percussion drilling system according to claim 14 wherein the slot is further characterized by a width of between one-thirty-second and one-sixteenth inch and stress relieving means comprising the innermost portion of the slot.

16. The percussion drilling system according to claim 15 wherein the slot comprises a saw cut formed in the head and comprising a plane extending parallel to but spaced from the longitudinal axis and a drilled hole extending transversely through the head at the lower end of the saw cut.

UNITED STATES PA'IE NT OFFICE CERTIFECATE OF CORRECTION Patent No. 3, 738,435 Dated June 12 1973 Invcncofls) Charles L. Bronson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

- "Assignee: Vorel Manufacturing Company" should be --Vare1 Manufacturing Company Column 3, line 52, before "by" insert -0n1y-- Signed and sealed this 2nd day of April 197A.

(SEAL) Attest:

EDWARD PLFLETCHERJR. C. MARSHALL DANN Attesting Officer Commissioner of Patents 

1. In a percussion earth boring system, a bit comprising a metal body having a longitudinal axis and including a shank and a head, said head comprising a substantially circular cylinder and including a portion secured to the shank and a bifurcated portion extending therefrom to a face disposed generally normal to the axis, said bifurcated portion having an axial length equal to between one-quarter and one-half of the diameter of the head.
 2. The percussion earth boring system according to claim 1 further including means for repeatedly impacting the bit into engagement with formations of the earth and thereby generating cuttings and wherein the distance separating the bifurcations of the head is sufficiently narrow that the cuttings cannot enter the slot extending therebetween.
 3. The percussion earth boring system according to claim 2 further including means for rotating the bit during the operation of the impacting means and a plurality of hardened inserts extending outwardly from the face and so arranged that upon rotation of the bit the inserts contact the entire bottom surface of the borehole.
 4. The percussion earth boring system according to claim 3 further including fluid course means for directing a fluid through outlets formed in the face and thereby remove cuttings from the bottom of the borehole.
 5. In a percussion bit for earth boring and the like, the combination of: a right circular cylindrical head havIng a longitudinal axis and terminating in a face disposed generally normal to the longitudinal axis, and a slot extending through the distal end of the head and parallel to the longitudinal axis, said slot being narrower in width than the diameter of the head by about two orders of magnitude and having a depth equal to between one-quarter and one-half of the diameter of the head.
 6. The combination according to claim 5 further including a plurality of hardened inserts extending outwardly from the face and so arranged that upon rotation and impacting of the head in a borehole the inserts are engaged with the entire bottom surface of the borehole.
 7. The combination according to claim 6 further comprising fluid course means formed in the head for directing a fluid into the bottom of the borehole and thereby removing cuttings from the borehole.
 8. The combination according to claim 7 wherein the slot divides the distal end of the head into proportions of substantially equal mass and wherein the innermost portion of the slot comprises stress relieving means.
 9. In a percussion bit, the combination of: a relatively soft metal body having a head portion, a tubular shank portion and a longitudinal axis, the head portion having a face disposed generally normal to the longitudinal axis; a plurality of hardened inserts secured in the head and projecting outwardly from the face, said inserts being arranged such that upon rotation of the head the inserts sweep the entire area of the face; and a slot formed in the head and extending parallel to but displaced from the longitudinal axis, said slot being sufficiently narrow to prevent the entry of cuttings of substantial size into the slot and having a total depth of between one-quarter and one-half the diameter of the head.
 10. The combination according to claim 9 wherein the slot formed in the head comprises a saw cut extending transversely through the head and wherein the innermost portion of the slot comprises stress relieving means.
 11. The combination defined in claim 10 further characterized by fluid course means extending through the head for directing fluid from within the tubular shank portion through outlets formed in the face and thereby removing cuttings from the bottom of a borehole.
 12. The combination according to claim 11 wherein the face includes an outwardly facing conically tapered peripheral portion and wherein at least a portion of the hardened inserts are positioned at spaced points around the peripheral portion for outward movement with the portions of the head on opposite sides of the slot to form the side walls of the borehole.
 13. A percussion drilling system comprising: a metal body having a longitudinal axis and including a right circular cylindrical head and a generally transversely extending face comprising the distal end of the head; a plurality of hardened inserts secured in the head and extending outwardly from the face, said inserts being so arranged that upon rotation of the head the inserts sweep the entire area of the face including the center; a slot extending axially into the head from the face to a depth of between one-quarter and one-half the diameter of the head and extending entirely through the head transversely thereof; and means for repeatedly impacting the head with the earth and simultaneously rotating the head and thereby forming a borehole by percussive engagement of the hardened inserts with formations of the earth, said slot being sufficiently narrow that intact portions of the formation tend not to enter the slot, whereby the impacting of the head causes the portions of the head on opposite sides of the slot to vibrate in a direction extending perpendicularly to the slot and to the axis so that the borehole is greater in diameter than the maximum diameter of the head.
 14. The percussion drilling system according to claim 13 wherein the slot is about two orders of magnitude narrower in width than tHe diameter of the head.
 15. The percussion drilling system according to claim 14 wherein the slot is further characterized by a width of between one-thirty-second and one-sixteenth inch and stress relieving means comprising the innermost portion of the slot.
 16. The percussion drilling system according to claim 15 wherein the slot comprises a saw cut formed in the head and comprising a plane extending parallel to but spaced from the longitudinal axis and a drilled hole extending transversely through the head at the lower end of the saw cut. 